A Norrish reaction in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
is a
photochemical reaction Organic photochemistry encompasses organic reactions that are induced by the action of light. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times ...
taking place with
ketones and
aldehydes
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions. The reaction is named after
Ronald George Wreyford Norrish
Ronald George Wreyford Norrish FRS (9 November 1897 – 7 June 1978) was a British chemist who was awarded the Nobel Prize in Chemistry in 1967.
Education and early life
Norrish was born in Cambridge and was educated at The Perse School and Emm ...
. While of limited synthetic utility these reactions are important in the
photo-oxidation of polymers
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidatio ...
such as
polyolefins,
polyesters
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
, certain
polycarbonates
Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily worke ...
and
polyketone
Polyketones are a family of high-performance thermoplastic polymers. The polar ketone groups in the polymer backbone of these materials gives rise to a strong attraction between polymer chains, which increases the material's melting point (255&nbs ...
s.
Type I
The Norrish type I reaction is the photochemical cleavage or
homolysis of aldehydes and ketones into two
free radical
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Ailments of unknown cause
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Biological processes
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Life extension
Metabo ...
intermediates (α-scission). The carbonyl group accepts a photon and is
excited to a photochemical
singlet state
In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number s=0. A ...
. Through
intersystem crossing Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity.
Excited Singlet and Triplet States
When an electron in a molecule with a singlet ground ...
the
triplet state
In quantum mechanics, a triplet is a quantum state of a system with a spin of quantum number =1, such that there are three allowed values of the spin component, = −1, 0, and +1.
Spin, in the context of quantum mechanics, is not a mechanical r ...
can be obtained. On cleavage of the
α-carbon
In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule.
Numeric locants
The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of ...
bond from either state, two radical fragments are obtained.
The size and nature of these fragments depends upon the stability of the generated radicals; for instance, the cleavage of
2-butanone
Butanone, also known as methyl ethyl ketone (MEK), is an organic compound with the formula CH3C(O)CH2CH3. This colourless liquid ketone has a sharp, sweet odor reminiscent of acetone. It is produced industrially on a large scale, but occurs in n ...
largely yields ethyl radicals in favor of less stable methyl radicals.
Several secondary reaction modes are open to these fragments depending on the exact molecular structure.
* The fragments can simply recombine to the original carbonyl compound, with
racemisation at the α-carbon.
* The acyl radical can lose a molecule of
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, forming a new carbon radical at the other α-carbon, followed by formation of a new carbon–carbon bond between the radicals.
The ultimate effect is simple extraction of the carbonyl unit from the carbon chain. The rate and yield of this product depends upon the
bond-dissociation energy
The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical ...
of the ketone's
α substituents. Typically the more α substituted a ketone is, the more likely the reaction will yield products in this way.
* The abstraction of an α-
proton from the carbonyl fragment may form a
ketene
In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). The name may also refer to the specific compound ethen ...
and an alkane.
* The abstraction of a β-proton from the alkyl fragment may form an
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
and an
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
.
![Norrish1](https://upload.wikimedia.org/wikipedia/commons/e/ec/Norrish1.png)
The synthetic utility of this reaction type is limited, for instance it often is a
side reaction
A side reaction is a chemical reaction that occurs at the same time as the actual main reaction, but to a lesser extent. It leads to the formation of by-product, so that the yield of main product is reduced:
: + B ->[] P1
: + C ->[] P2
P1 is th ...
in the Paternò–Büchi reaction. One organic synthesis based on this reaction is that of bicyclohexylidene.
Type II
A Norrish type II reaction is the photochemical
intramolecular abstraction of a
γ-hydrogen (a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-
biradical as a primary photoproduct. Norrish first reported the reaction in 1937.
Secondary reactions that occur are fragmentation (β-scission) to form an
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
and an
enol
In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). The t ...
(which will rapidly
tautomer
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hyd ...
ise to a carbonyl), or intramolecular recombination of the two radicals to a substituted
cyclobutane
Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercia ...
(the Norrish–Yang reaction).
Scope
The Norrish reaction has been studied in relation to
environmental chemistry
Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source. It can be defined as ...
with respect to the
photolysis of the
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
heptanal
Heptanal or heptanaldehyde is an alkyl aldehyde. It is a colourless liquid with a strong fruity odor, which is used as precursor to components in perfumes and lubricants.
Production
The formation of heptanal in the fractional distillation of cas ...
, a prominent compound in Earth's atmosphere. Photolysis of heptanal in conditions resembling atmospheric conditions results in the formation of
1-pentene and
acetaldehyde in 62%
chemical yield
In chemistry, yield, also referred to as reaction yield, is a measure of the quantity of moles of a product formed in relation to the reactant consumed, obtained in a chemical reaction, usually expressed as a percentage. Yield is one of the pr ...
together with cyclic alcohols (
cyclobutanol
Cyclobutanol is an organic compound with the chemical formula C4H8O. It contains a cyclobutyl group with a hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom ...
s and
cyclopentanol
Cyclopentanol or cyclopentyl alcohol is a cyclic alcohol. It is also known as hydroxycyclopentane.
Reactions
The dehydration of cyclopentanol produces cyclopentene
Cyclopentene is a chemical compound with the formula . It is a colorless liqu ...
s) both from a Norrish type II channel and around 10% yield of
hexanal from a Norrish type I channel (the initially formed n-hexyl radical attacked by oxygen).
In one study the photolysis of an
Acyloin
Acyloins or α-hydroxy ketones are a class of organic compounds which all possess a hydroxy group adjacent to a ketone group. The name acyloin is derived from the fact that they are formally derived from reductive coupling of carboxylic acyl grou ...
derivative in water in presence of
hydrogen tetrachloroaurate (HAuCl
4) generated
nanogold particles with 10
nanometer diameter. The species believed to responsible for reducing Au
3+ to Au
0 is the Norrish generated
ketyl
A ketyl group in organic chemistry is an anion radical that contains a group R2C−O•. It is the product of the 1-electron reduction of a ketone.
Another mesomeric structure has the radical position on carbon and the negative charge on oxyge ...
radical.
Paquette's 1982 synthesis of
dodecahedrane
Dodecahedrane is a chemical compound, a hydrocarbon with formula , whose carbon atoms are arranged as the vertices (corners) of a regular dodecahedron. Each carbon is bound to three neighbouring carbon atoms and to a hydrogen atom. This compound ...
involves three separate Norrish-type reactions in its approximately 29-step sequence.
An example of a synthetically useful Norrish type II reaction can be found early in the total synthesis of the biologically active
cardenolide
A cardenolide is a type of steroid. Many plants contain derivatives, collectively known as cardenolides, including many in the form of cardenolide glycosides (cardenolides that contain structural groups derived from sugars). Cardenolide glycoside ...
ouabagenin by
Baran and coworkers.
The optimized conditions minimize side reactions, such as the competing Norrish type I pathway, and furnish the desired intermediate in good yield on a multi-gram scale.
See also
*
Photo-Fries rearrangement - a related reaction of aromatic carbonyls
*
McLafferty rearrangement The McLafferty rearrangement is a reaction observed in mass spectrometry during the fragmentation or dissociation of organic molecules. It is sometimes found that a molecule containing a keto-group undergoes β-cleavage, with the gain of the γ-hyd ...
- similar to a Type II Norrish reaction. Caused by electron impact ionization rather than light
*
Carbon monoxide-releasing molecules
Carbon monoxide-releasing molecules (CORMs) are chemical compounds designed to release controlled amounts of carbon monoxide (CO). CORMs are being developed as potential therapeutic agents to locally deliver CO to cells and tissues, thus overcomin ...
References
{{DEFAULTSORT:Norrish Reaction
Free radical reactions
Photochemistry
Name reactions